feat(subscriptions): dry-run backfill preview — B4 preview (plan #708)

Owning the walk lets an operator gauge "is this source worth a backfill?" before
arming one. ingest_core.Ingester.preview walks the first few feed pages and
counts media NOT already in the seen/dead ledgers, downloading nothing
(read-only). download_backends.preview_source resolves the campaign id + runs it
(native-only, mirrors verify_source_credential / run_download); POST
/api/sources/{id}/preview returns {total_new, posts_scanned, has_more, sample[]}
(409 on auth/drift/unresolvable, 400 for gallery-dl platforms). PatreonClient
gains post_meta(post) for the sample's title/date.

UI: a Patreon-only Preview button (mdi-eye-outline) on SourceRow + SourceCard
opens PreviewDialog — self-fetches with loading / error / empty / result states
and a "Start backfill" shortcut. Store action previewSource.

Tests: preview counts new media without downloading + samples only posts with
new items; page_limit caps the walk + flags has_more.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/SourceRow.vue

@@ -69,6 +69,16 @@
             : 'Backfill — walk the full post history until complete' }}
         </v-tooltip>
       </v-btn>
+      <v-btn
+        v-if="source.platform === 'patreon' && source.backfill_state !== 'running'"
+        size="x-small" variant="text"
+        @click.stop="$emit('preview', source)"
+      >
+        <v-icon>mdi-eye-outline</v-icon>
+        <v-tooltip activator="parent" location="top">
+          Preview — count what a backfill would download (no download)
+        </v-tooltip>
+      </v-btn>
       <v-btn
         v-if="source.platform === 'patreon' && source.backfill_state !== 'running'"
         size="x-small" variant="text"
@@ -106,7 +116,7 @@ const props = defineProps({
   checking: { type: Boolean, default: false },
   warningThreshold: { type: Number, default: 5 },
 })
-const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover'])
+const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover', 'preview'])
 
 function onToggleEnabled(value) {
   emit('toggle', { source: props.source, enabled: value })